The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean region and almost completely enclosed by land. Eleven European countries border the Mediterranean Sea: Spain, France, Monaco, Italy, Malta, Slovenia, Croatia, Bosnia and Herzegovina, Montenegro, Albania, and Greece. The Mediterranean people have traditionally lived by farming, but many of these countries now have thriving industries. Though the climate around the Mediterranean is much warmer than that of northern Europe, winters can still be chilly.
The seven demigods journey through the Mediterranean Sea on their quest to save Nico di Angelo and on their search for the Mark of Athena, the Athena Parthenos, using a coin given to Annabeth Chase by her mother, Athena. They had to get permission from Hercules to pass through the Pillars of Hercules. In the Mare Nostrum, they encounter a hybrid shrimp fish monster sent by Keto, the Stymphalian Birds, evil venti (wind spirits), a wild pegasus, and Ichthyocentaurs who are Chiron's half-brothers who trains mer-heroes at Camp Fish-Blood. They also encounter Chrysaor, a pirate in the sea and Percy's half-brother. The Seven battle him and eventually win, by scaring away his pirate crew by pretending that Dionysus is on board the ship. They surround him and Frank Zhang transforms into a grizzly bear and pushes Chrysaor into the sea. After they win they use the pirate's boat filled with six million in gold as tribute to the god, Bacchus/Dionysus.
- Romans call it the Mare Nostrum (Latin for "Our Sea").
- The Mediterranean Sea was off limits to Roman demigods since the past few centuries because it was the ancestral home of monsters, Titans, Giants, and the gods, and it was considered "dangerous".
- The name Mediterranean is derived from the Latin, mediterraneus, meaning "inland" or "in the middle of the land" (from medius, "middle" and terra, "land").